Decubitus ulcers (also known as pressure ulcers) afflict many people. Those who spend long periods of time on support surfaces such as wheelchairs and beds are particularly vulnerable.
The sitting position of a wheelchair user focuses significant loads on a small area; namely, the buttocks and surrounding areas. Carlson J M, Payette M J, Vervena L P, “Seating orthosis design for prevention of decubitus ulcers,” J Prosth & Orth, Spring 1995; 7(2): 51-60. Wheelchair users often sit for prolonged periods; moreover, the physiology of many wheelchair users (e.g., geriatric or paraplegic) makes them more prone to ulcer formation. Bennett L, Kavner D, Lee B Y, Trainor F S, Lewis J M, “Skin stress and blood flow in sitting paraplegic patients,” Arch Phys Med Rehabil., April 1984; 65(4):186-90. With regard to people with spinal cord injury (SCI)—most of whom use wheelchairs—between a third and a half develop ulcers within five years after the injury. Five to seven percent of people with SCI eventually require hospitalization due to decubitus ulcers and seven to eight percent eventually die of complications from ulcers. Agram L, Gefen A, “Pressure ulcers and deep tissue injury: a bioengineering perspective,” Journal of Wound Care, Vol. 16, Iss. 8, 1 Sep. 2007, pp 336-342.
Overall, the incidence rates for all kinds of decubitus ulcers range from 0.4% to 38% in acute care, 2.2% to 23.9% in long-term care, to 0% to 17% in home care. More than 2.5 million pressure ulcers are treated each year in the United States. Estimates put United States expenditures on the treatment of decubitus ulcers as high as $11 billion. Reddy M, Gill S S, Rochon P A, “Preventing pressure ulcers: a systematic review,” JAMA, 23 Aug. 2006; 296(8): 974-84.
Most decubitus ulcers form over weightbearing bony prominences. In seating, the most frequently involved areas are over the sacrum, coccyx, ischial tuberosities and greater trochanters. Carlson et al., Spring 1995. For people in beds, other involved areas can include those over the back of the heels, the back of the head, the elbows, and the shoulder blades, for example.
Two forms of external loading play a role in the formation of decubitus ulcers: pressure and shear. Friction forces act parallel (or tangential) to the skin surface and produce shear strains within the skin and underlying tissue. Both pressure and shear harm skin. Carlson et al., Spring 1995.
For many years, care providers focused predominantly on alleviating pressure when evaluating support surfaces and wheelchair cushions. Reducing pressure is accomplished by redistributing the overall contact pressure. Carlson et al., Spring 1995. This commonly involves off-loading pressure from a vulnerable area to a less vulnerable area.
Like pressure, shear is also reduced by lowering peak pressure because shear is caused by two phenomena: pressure and friction. In the context of seat cushions and other support surfaces, there are several reasons to focus on shear reduction. First, in relative terms, shear is more destructive of tissue integrity than pressure. Bennett et al., April 1984. Second, in some instances, it may be easier to manage friction and shear than it is to manage pressure. Third, most efforts to control peak pressure involve foam materials that can accelerate ulcer formation by impeding heat dissipation and evaporation.